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Flow control and reduced-order modelling of transition in shear flows
KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis direct numerical simulation is used to investigate the possibility to delay the transition from a laminar to a turbulent flow in boundary layer flows. Furthermore, modal analysis techniques are used to identify the coherent structures in wind turbine wake.

Among different transition scenarios, the classical transition scenario is considered where the laminar-turbulent transition occursdue to Tollmien-Schlichting (TS) waves. These waves are convectively unstable and when triggered inside the boundary layer, they grow exponentially inamplitude as they are advected downstream of the domain. The aim is to suppressthese waves using flow control strategies based on a row of spatially localised sensors and actuators distributed near the wall inside the boundary layer. To avoid the high dimensionality arising from discretisation of the Navier–Stokes equations, a reduced order model (ROM) based on the Eigensystem Realisation Algorithm(ERA) is obtained and based on that a linear controller is designed. To manip-ulate the flow, a plasma actuator is modelled and implementedas an externalforcing. To account for the restrictions of the plasma actuators, several strategies are proposed and tested within the LQG framework. We studied also the design of a faster controller based on decentralised approach and compared the performance to a more expensive centralised controller. The outcomes revea lsuccessful performance in mitigating the energy of the disturbances inside the boundary layer and suppressing the TS waves.

To extract coherent features of the wind turbine wakes, modal decomposition techniques are employed. In this method a large dynamical system is reduced to a fewer number of degrees of freedom. Two decomposition techniques are employed, namely proper orthogonal decomposition and dynamic mode decomposition. In the former, the flow is decomposed into a set of orthogonal structures which are ranked according to their energy contents in a hierarchical manner. In the latter, the eigenvalues and eigenvectors of the underlying approximate linear operator of the system is evaluated. In particular each mode is associated with a specific frequency and growth rate. The results revealed the coherent structures which are dynamically significant for the onset of instability in the wind turbine wake

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. , vii, 57 p.
Series
TRITA-MEK, ISSN 0348-467X ; 2014:14
Keyword [en]
Flow control, plasma actuator, wing, leading edge, flat plate, wind turbine, optimal controller, model reduction, proper orthogonal decomposition, dynamic mode decomposition.
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-145631ISBN: 978-91-7595-170-6 (print)OAI: oai:DiVA.org:kth-145631DiVA: diva2:719319
Public defence
2014-06-10, Sal F3, Lindstedtsvägen 26. KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Note

QC 20140526

Available from: 2014-05-26 Created: 2014-05-23 Last updated: 2014-05-26Bibliographically approved
List of papers
1. Output Feedback Control of Blasius Flow with Leading Edge Using Plasma Actuator
Open this publication in new window or tab >>Output Feedback Control of Blasius Flow with Leading Edge Using Plasma Actuator
2013 (English)In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 51, no 9, 2192-2207 p.Article in journal (Refereed) Published
Abstract [en]

The evolution and control of a two-dimensional wave packet developing on a flat plate with a leading edge is investigated by means of direct numerical simulation. The aim is to identify and suppress the wave packets generated by freestream perturbations. A sensor is placed close to the wall to detect the upcoming wave packet, while an actuator is placed further downstream to control it. A plasma actuator is modeled as an external forcing on the flow using a model based and validated on experimental investigations. A linear quadratic Gaussian controller is designed, and an output projection is used to build the objective function. Moreover, by appropriate selection of the proper orthogonal decomposition modes, we identify the disturbances to be damped. A reduced-order model of the input-output system is constructed by using system identification via the eigensystem realization algorithm. A limitation of the plasma actuators is the unidirectional forcing of the generated wall jet, which is predetermined by the electrodes' location. In this paper, we address this limitation by proposing and comparing two different solutions: 1) introducing an offset in the control signal such that the resulting total forcing is oriented along one direction, and 2) using two plasma actuators acting in opposite directions. The results are compared with the ideal case where constraints are not accounted for the control design. We show that the resulting controllers based on plasma actuators can successfully attenuate the amplitude of the wave packet developing inside the boundary layer.

Keyword
Eigensystem realization algorithms, Input-output systems, Objective functions, Output feedback controls, Proper orthogonal decompositions, Quadratic Gaussian, Reduced order models, Two-dimensional waves
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-129100 (URN)10.2514/1.J052141 (DOI)000323589400012 ()2-s2.0-84882933777 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20130920

Available from: 2013-09-20 Created: 2013-09-19 Last updated: 2017-12-06Bibliographically approved
2. Feedback Control for Laminarization of flow over Wings
Open this publication in new window or tab >>Feedback Control for Laminarization of flow over Wings
2015 (English)In: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 94, no 1, 43-62 p.Article in journal (Refereed) Published
Abstract [en]

An active control strategy is implemented to attenuate the amplitude of the Tollmien-Schlichting (TS) waves inside the boundary layer of an airfoil. The dynamics of the system are modelled by the linearised Navier-Stokes equations. The impulse response to an initial disturbance, initially located outside of the boundary layer and in front of the airfoil is considered. The perturbation evolves and penetrates inside the boundary layer and triggers the TS waves. Different control strategies including the linear quadratic Gaussian (LQG) and model predictive control (MPC) are designed based on a reduced order model where the sensors and actuators are localised near the wall. An output projection is used to identify the unstable disturbances; the objective function of the controller is selected as a set of proper orthogonal decomposition (POD) modes; to isolate the dynamics of the TS waves, the modes with high energy contents in the TS wave frequency band are considered as the objective of the controller. A plasma actuator is modelled and implemented as an external forcing on the flow. To account for the limitations of the plasma actuator several strategies are examined and the results are compared with a classical LQG controller. The outcomes reveal successful performance in mitigating the amplitude of the wavepacket developing inside the boundary layer.

Keyword
Active Flow Control, Boundary Layer, Tollmien-Schlichting wavepacket
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-145659 (URN)10.1007/s10494-014-9578-9 (DOI)000347407100003 ()
Note

QC 20150209. Updated from manuscript to article in journal.

Available from: 2014-05-26 Created: 2014-05-26 Last updated: 2017-12-05Bibliographically approved
3. Control of instabilities in boundary layer of unswept wing
Open this publication in new window or tab >>Control of instabilities in boundary layer of unswept wing
(English)Manuscript (preprint) (Other academic)
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-145660 (URN)
Note

QS 2014

Available from: 2014-05-26 Created: 2014-05-26 Last updated: 2014-05-26Bibliographically approved
4. Centralised Versus Decentralised Active Control of Boundary Layer Instabilities
Open this publication in new window or tab >>Centralised Versus Decentralised Active Control of Boundary Layer Instabilities
2014 (English)In: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 93, no 4, 537-553 p.Article in journal (Refereed) Published
Abstract [en]

We use linear control theory to construct an output feedback controller for the attenuation of small-amplitude three-dimensional Tollmien-Schlichting (TS) wavepackets in a flat-plate boundary layer. A three-dimensional viscous, incompressible flow developing on a zero-pressure gradient boundary layer in a low Reynolds number environment is analyzed using direct numerical simulations. In this configuration, we distribute evenly in the spanwise direction up to 72 localised objects near the wall (18 disturbances sources, 18 actuators, 18 estimation sensors and 18 objective sensors). In a fully three-dimensional configuration, the interconnection between inputs and outputs becomes quickly unfeasible when the number of actuators and sensors increases in the spanwise direction. The objective of this work is to understand how an efficient controller may be designed by connecting only a subset of the actuators to sensors, thereby reducing the complexity of the controller, without comprising the efficiency. If n and m are the number of sensor-actuator pairs for the whole system and for a single control unit, respectively, then in a decentralised strategy, the number of interconnections deceases mn compared to a centralized strategy, which has n (2) interconnections. We find that using a semi-decentralized approach - where small control units consisting of 3 estimation sensors connected to 3 actuators are replicated 6 times along the spanwise direction - results only in a 11 % reduction of control performance. We explain how "wide" in the spanwise direction a control unit should be for a satisfactory control performance. Moreover, the control unit should be designed to account for the perturbations that are coming from the lateral sides (crosstalk) of the estimation sensors. We have also found that the influence of crosstalk is not as essential as the spreading effect.

Keyword
Flow control, Boundary layer flows, Hydrodynamic stability, Control theory, Model reduction
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-145661 (URN)10.1007/s10494-014-9552-6 (DOI)000345076500001 ()2-s2.0-84920259408 (Scopus ID)
Note

QC 20141215 Updated from manuscript to article in journal.

Available from: 2014-05-26 Created: 2014-05-26 Last updated: 2017-12-05Bibliographically approved
5. Mutual inductance instability of the tip vortices behind a wind turbine
Open this publication in new window or tab >>Mutual inductance instability of the tip vortices behind a wind turbine
Show others...
2014 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 755, 705-731 p.Article in journal (Refereed) Published
Abstract [en]

Two modal decomposition techniques are employed to analyse the stability of wind turbine wakes. A numerical study on a single wind turbine wake is carried out focusing on the instability onset of the trailing tip vortices shed from the turbine blades. The numerical model is based on large-eddy simulations (LES) of the Navier-Stokes equations using the actuator line (ACL) method to simulate the wake behind the Tj ae reborg wind turbine. The wake is perturbed by low-amplitude excitation sources located in the neighbourhood of the tip spirals. The amplification of the waves travelling along the spiral triggers instabilities, leading to breakdown of the wake. Based on the grid configurations and the type of excitations, two basic flow cases, symmetric and asymmetric, are identified. In the symmetric setup, we impose a 120 degrees symmetry condition in the dynamics of the flow and in the asymmetric setup we calculate the full 360 degrees wake. Different cases are subsequently analysed using dynamic mode decomposition (DMD) and proper orthogonal decomposition (POD). The results reveal that the main instability mechanism is dispersive and that the modal growth in the symmetric setup arises only for some specific frequencies and spatial structures, e.g. two dominant groups of modes with positive growth (spatial structures) are identified, while breaking the symmetry reveals that almost all the modes have positive growth rate. In both setups, the most unstable modes have a non-dimensional spatial growth rate close to pi/2 and they are characterized by an out-of-phase displacement of successive helix turns leading to local vortex pairing. The present results indicate that the asymmetric case is crucial to study, as the stability characteristics of the flow change significantly compared to the symmetric configurations. Based on the constant non-dimensional growth rate of disturbances, we derive a new analytical relationship between the length of the wake up to the turbulent breakdown and the operating conditions of a wind turbine.

Keyword
instability, vortex interaction, wakes
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-145662 (URN)10.1017/jfm.2014.326 (DOI)000341128600036 ()
Funder
Swedish e‐Science Research Center
Note

QC 20140930. Updated from manuscript to article in journal.

Available from: 2014-05-26 Created: 2014-05-26 Last updated: 2017-12-05Bibliographically approved

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